Study on Sound Propagation Performance of Mechanized Coal Gangue-Fine Sand Filling Body

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Introduction
For some coal veins overlaid unique stratigraphic structures, the mining conditions are technically tricky due to the complexity and variability of the strata. For example, in the sharply inclined coal seams mining stage, the overburden displacement characteristics encountered are hypothesized and calculated by combining the long-walled comprehensive mining workings in medium-thick coal seams, and the overburden transport law obtained is in good agreement with the feld conditions [1][2][3]. In the coal seam damage process in coal mining working face under signifcant mining height conditions, the researchers analyzed the force characteristics of large coal walls based on feld observation and theoretical research. Tey proposed feasible control measures for reducing coal wall destabilization damage [4,5]. In the various stages of coal mining in the plains and mountains of diferent scales, many empty areas are produced because coal is continuously mined out, and the mechanical function of the surrounding rock body will gradually deteriorate with time. Te stress on the rock layer will be redistributed with coal mining. During development of a particular stage, the rock body will be sharply displaced towards the mining area, and then the surrounding rock will collapse and cause the surface collapse, which often disturbs the average production of coal enterprises and brings a series of safety and social problems, so some researchers have carried out analysis for the surface collapse problem, using synthetic aperture radar to observe the surface collapse, summarize the law and take specifc measures, and analyze the data, which provide an accurate early warning and judgment decision. [6,7].
For the ground subsidence caused by coal mining, the Quang Ninh coal mine in Vietnam used Sentinel-1 radar observation technology to monitor and predict the gradual development of subsidence in real-time, and the data obtained can be used to guide subsequent disaster prevention and mitigation decisions [8]. In order to cope with the great danger brought by the collapse of the roof rock in the mining area, relevant researchers have conducted a lot of feld tests and theoretical analysis. Te use of solid waste to fll the goaf has an excellent efect on the prevention and control of surface collapse in the maintenance of roof strata. For the study of the mechanical characteristics of the use of gangue-flled quarry areas, the researchers analyzed the force situation of loose gangue under waterlogging conditions while investigated the efect of diferent factors on the deformation and crushing characteristics of loose gangue flling materials. Te conclusions provide a theoretical basis for applying loose gangue flling [9]. In recent years, related research has involved the cementation law of colloidal gangue-fy ash fller, and the complex interaction between pressure and thermodynamics is considered comprehensively in mechanical testing. Te THMC coupling model with high applicability is successfully established [10]. In flling mining to control rock dynamics disaster and sandbased cemented fller performance optimization, the researchers summarized the laws of surrounding rock movement in underground and open-pit mining in combination with site conditions. Tey improved the performance index of the fller by optimizing the combination of flling materials [11][12][13][14].
In the area of compaction and homogeneity testing of porous materials such as engineering rock and concrete, researchers have explored the correlation between acoustic properties and strength, density, and other indicators using the acoustic propagation properties of materials, and the fndings can be used to guide material performance analysis [15,16]. Regarding concrete and rock quality inspection techniques, researchers combine the sound propagation properties of materials to compare the similarities and diferences between intact and defective materials, thus making reasonable judgments about material integrity [17,18]. Among the methods for integrity testing of various types of cemented and uncemented fllings, researchers have used the mechanical characteristics of poor concrete-like materials to obtain the sound propagation pattern of fllings with the help of conventional concrete material testing methods [19][20][21][22]. Gangue, as a flling aggregate, has its characteristics. Trough analyzing the gangue flling body pressure process of acoustic emission performance, researchers explore the technical ways to mitigate the flling body from damage [23,24]. Soft rock mine has low strength of surrounding rocks and poor overall integrity of local rock masses. In the continuous mining of soft rock coal mines, high-quality flling treatment is required for the extraction area. Applying the acoustic velocity measurement method to the nondestructive testing of the quality of the flling body is an excellent way to efectively maintain the stability of the rock formation.
In this paper, the acoustic characteristic analysis of mechanism gangue-sand flling body is performed to seek relationship between the strength of flling body and parameters of acoustic velocity to provide a viable technical basis for flling material performance. Te bulk coal gangue as flling aggregate is applied to the flling management of the mining area, which can make the most important solid waste of coal mining enterprises resource utilization, thus creating good economic benefts and ecological and environmental benefts for the enterprise, while minimizing the harm caused by the solid waste containing harmful elements.

Mechanism Coal Gangue.
Te mechanism gangue used in the test was taken from a coal mine in Pu'an County, Liupanshui City, Guizhou Province. Using a small jaw crusher to mechanically crush large pieces of coal gangue, according to the characteristics of the flling process technology, the maximum aperture of the discharge screen of the jaw crusher is 10 mm. Te black block gangue is crushed into 0∼10 mm. Trough the relevant geotechnical performance tests and principles of calculation, the performance of the crushed fne-grained gangue is shown in Table 1.

Fine Sand.
In order to facilitate the collection of test materials, the fne sand is conventional yellow sand for construction. Its main component is siliceous quartz sand, whose properties are shown in Table 2.

Cement.
Te PO 42.5-grade ordinary silicate cement is used as the flling cementitious material. Te physical performance index is measured; the results are shown in Table 3.

Water.
Te mixed water of the flling slurry is tap water coming from Xiangtan City Water Works. Te pH value of water is 7.13.

Preparation of Filling Slurry
Te slump test of flling slurry with diferent mix proportion was conducted. Based on the slurry fow characteristics, the appropriate aggregate ash ratio (AA) (gangue and fne sand to cement mass ratio), flling slurry concentration (SC), and gangue sand mass ratio (GS) were determined. 3 group tests of diferent mix proportions were performed. Every group has 3 flling slurry concentration, as shown in Table 4.
According to the requirements of coal mine flling technology, while ensuring pump pressure delivery, it is necessary to minimize the water consumption of the flling slurry to reduce the negative impact of water on the underground mining rock mass. In terms of the mechanical and fow properties of the flling material, considering the connection between mining and flling during coal mining, it is necessary to accelerate the progress of the mining and flling cycle. Terefore, while fully utilizing self-produced solid waste, it is necessary to improve the early strength of the flling material and adopt a material formula that is conducive to improving the fuidity of the flling material slurry and the early strength of the flling material. Te slurry concentration (SC) of the three groups are, respectively, 86%, 84%, and 82%, and the aggregate ash ratio (AA) is 3.5 : 1. Te gangue sand ratio (GS) is 3 : 7, 5 : 5, and 7 : 3. According to the mix proportion in Table 5, the flling slurry was prepared   2 Shock and Vibration in the laboratory and then square specimens of 70.7 × 70.7 × 70.7 mm was formed. After the specimens reach the fnal set state, they are remolded and placed in the laboratory maintenance box for regular moisture maintenance.

Sonic Test Instrument.
Ultrasonic technology has obvious unique advantages in nondestructive test. In this paper, the used instrument is a multifunctional acoustic wave tester, as shown in Figure 1. Te main components of the instrument include the host, transducer, transmission line, and power supply. Te advantages of the instrument are as follows [25,26].
(1) Developed and researched based on WINDOWS system, parameter setting, and data acquisition controlled by computer (2) Combined with the LABVIEW virtual instrument development platform, it can meet various needs of users in diferent situations (3) Te instrument hardware is highly integrated, all components contained in a unique toolbox, which is easy to carry and store (4) Te instrument's software also has various functions, such as waveform display and spectrum analysis, providing excellent convenience for subsequent data processing In order to accurately measure the longitudinal wave velocity of the flling body, in this acoustic wave test, the multifunctional acoustic wave tester was routinely calibrated. Specifc main parameters were also set according to the applicable conditions and material properties, as shown in Table 5.

Acoustic Test Principle and Test
Device. Te working principle of the multifunction acoustic wave tester is that a high-voltage pulse generator generates a voltage signal, then digitized by an A/D converter, making it evident through an amplifer, and fnally transmitted to the sampling      [27,28]. Te receiver receives the analog signal in transmission. It is digitized in an A/D converter, followed by a data collector that amplifes it and automatically collects and stores it in the instrument.
Following the acoustic wave test procedure, the preparation of the acoustic propagation velocity test of the cemented flling body was carried out. When using butter as a coupling agent, it is necessary to prepolish the flling specimens with rough ends to avoid the negative efects caused by uneven contact surfaces. During the test, the butter was added between the transducer and the specimen to eliminate the air bubbles, so that there is good contact between the transducer and the specimen. Te test result is thus more accurate [29,30]. Te flling body acoustic feld test is shown in Figure 2

Variation Law of Longitudinal Wave Velocity.
Te original data of the acoustic wave test of the flling body obtained in this test are processed on the Origin software to restore the test waveform. Te No. 3 specimen with a curing age of 5 d is taken as an example, as shown in Figure 3. Use the origin software function to check the travel time Δt of the longitudinal wave in the flling body, and then use equation (1) to calculate the longitudinal wave velocity V p : Te results of the acoustic wave velocity test are shown in Table 6. It can be seen that the maximum longitudinal wave velocity of the flled body test is 2.413 km/s and the minimum is 1.556 km/s. According to further analysis, the wave velocity change of the flling body during the solidifcation and hardening process (5 d∼10 d curing age) was obtained, as shown in Figure 4. It can be seen that the wave velocity has increased with the curing age for three groups. With advance of hydration of cement, the water and air in the specimen are continuously reduced. Te original pores are gradually flled and compacted by the hydration products and the compactness increases. Tus, the longitudinal wave velocity of the specimen shows an increasing trend.

Relationship between Longitudinal Wave Velocity and
Strength. According to the acoustic test of the mechanism gangue-fne sand flled specimen, there is a certain regularity between the longitudinal wave velocity and uniaxial compressive strength, as shown in Figure 5. It can be seen that the uniaxial compressive strength has a positive correlation with acoustic wave velocity. After using exponential, linear, logarithmic, and polynomial functions to analyze the data, it was fnally found that a good ftting result could be obtained by using the four-term function, and the correlation coefcient R 2 reached above 0.98. Tis shows that it is accurate and reasonable to predict the uniaxial compressive strength by using the longitudinal wave velocity of the acoustic wave. Te equation is shown in equation (2), and the parameters B 1 , B 2 , B 3 , and B 4 in the corresponding analytical expressions have the corresponding optimal values, as shown in Table 7.
where S up is the uniaxial compressive strength of the backfll specimen and V up is the longitudinal wave velocity. Te B ₁ , B ₂ , B ₃ , and B ₄ , are coefcients related to the flling body.

Efects of Slurry Concentration on Longitudinal Wave
Velocity. Te relationship curve between the longitudinal wave velocity and the slurry concentration (SC) is shown in Figure 6. For gangue-sand ratio (GS) of 3 : 7, when the maintenance age is 5 d and 6 d, the slurry concentration (SC) and the strength of the flling body are not signifcantly   correlated. Te flling body with slurry concentration (SC) of 84% has the highest strength. When the maintenance age exceeds 6 d, the strength of the flling body with the slurry concentration (SC) increases; the strength of the flling body prepared at the 86% slurry concentration (SC) is the highest. It indicates that the early strength of the flling body is low at a low gangue ratio (GS) and high slurry concentration (SC), but later it has good growth. When there is sufcient time on site, a lower gangue ratio (GS) can be used to prepare the mechanism coal gangue-fne sand backfll material to fll and treat goaf in mine.
In addition, for 5 : 5 or 7 : 3 gangue-sand ratio (GS), it can be seen that almost all acoustic longitudinal wave velocity increases with the slurry concentration (SC) in each curing age, which indicates that in the condition of larger ganguesand ratio (GS), for the same hydration reaction time, the strength of flling body increases with the slurry concentration (SC). In the subsequent experimental test, the optimal flling material formulation can be sought in the higher gangue-sand ratio (GS) flling material.
In this test, no measures and means were taken to reduce and eliminate the relevant efects, so individual discrepancy

Conclusions
In this study, the fller specimens with diferent slurry concentration (82, 84, and 86%) and gangue-sand ratio (GS) were made. Trough the acoustic wave test, the law of longitudinal wave velocity in the flling body at diferent curing ages was analyzed, and the quartic function relationship between the backfll body strength and the longitudinal wave velocity is established.
Gangue ratio (GS), slurry concentration (SC), and curing time (t) are all crucial factors that afect the uniaxial compressive strength of mechanism coal gangue-fne sand backfll. Te strength of the backfll increases with the increase of slurry concentration (SC) and curing time (t). When the sand-cement ratio (C) remains unchanged, under the same slurry concentration (F) and curing time (t), the uniaxial compressive strength of the specimen shows a negative growth trend with the increase of the gangue ratio (GS).
By comparing the compressive strength of the mechanism gangue-fne sand backfll with its longitudinal wave velocity data, it is found that the higher the uniaxial compressive strength, the faster the longitudinal wave speed of   Te longitudinal wave velocity of the flling body was the fastest when the slurry concentration (SC) was 84% before 6 d when the gangue-sand ratio (GS) was 3 : 7. However, the longitudinal wave velocity was the fastest when the slurry concentration (SC) was 86% after 6 d. Tat is to say, when the gangue-sand ratio (GS) is small and the slurry concentration (SC) is large, the early strength of the prepared mechanism gangue-fne sand flling body is low, and its later growth is good. At the same time, it can be seen that the flling body longitudinal wave velocity increases with the increase of slurry concentration (SC).
In studying sound propagation characteristics of concrete-like materials, gangue concrete's mechanical and acoustic emission characteristics under uniaxial compression were analyzed [31]. Due to the special lithology of coal gangue, it has high porosity and more microscopic cracks, which lead to its low mechanical properties. Tus the strength of coal gangue concrete is lower than that of gravel concrete. Usually, the maximum load-bearing strength of gangue concrete is 35∼40 MPa, which is much higher than the strength of the mechanism gangue-fne sand flling body, so in the subsequent study, the relationship between higher strength flling body and its acoustic wave can be investigated. At the same time, in the follow-up test, the gangue can be considered compared to other general flling material characteristics in the acoustic testing process to seek the most optimal solution to reduce adverse efects caused by the lithology of the gangue, porosity, and microscopic fractures of the flling body.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.